The present invention relates to a semiconductor light-emitting element.
In recent years, semiconductor light-emitting elements are widely used in an outdoor display, automobile indicator, and the like. The semiconductor light-emitting element is a device using emission recombination of electrons and holes injected in a p-n junction region. Emission ranging from infrared radiation to ultraviolet radiation can be realized by changing the semiconductor material of a light-emitting layer.
FIG. 30 shows the structure of a conventional semiconductor light-emitting element. An n-type GaAs buffer layer 3202, an n-type DBR (Distributed Bragg Reflector) reflective layer 3203 made of InGaAlP and GaAs to reflect light using the Bragg reflection effect, an n-type InGaAlP cladding layer 3204, an active layer 3205, a p-type InGaAlP cladding layer 3206, a p-type AlGaAs window layer 3207, and a p-type GaAs contact layer 3208 are sequentially formed on the upper surface of an n-type GaAs substrate 3201.
An n-type electrode 3209 is formed on the lower surface of the n-type GaAs substrate 3201, and a p-type electrode 3210 is formed on the p-type GaAs contact layer 3208. Power is supplied to the light-emitting element to emit light from the active layer 3205. Light emitted downward in FIG. 30 by the active layer 3205 is reflected by the reflective layer 3203, and radiated to above the element via the window layer 3207 together with the light emitted upward.
The conventional semiconductor light-emitting element suffers the following problem.
Part of light that is emitted downward by the active layer 3205 and travels straight toward the reflective layer 3203 is reflected by the reflective layer 3203 without being absorbed by the substrate 3201, and can be effectively extracted outside.
However, the reflective layer 3203 exhibits a very low reflectivity with respect to light traveling diagonally toward the reflective layer 3203, so not all the light from the active layer 3205 can be extracted outside.
The semiconductor light-emitting element absorbs light by a substrate which provides a critical angle defined by the difference in refractive index between the semiconductor crystal and the atmosphere or enables crystal growth. For this reason, light which can be extracted outside is only several % of internally emitted light.
FIG. 26 shows the structure of another semiconductor light-emitting element relating to the present invention.
A multilayered reflective film 1001, p-type contact layer 1002, p-type cladding layer 1003, active layer 1004 functioning as a light-emitting layer, n-type cladding layer 1005, and n-type contact layer 1006 are formed on a p-type semiconductor substrate 1000. An n-type electrode 1007 is formed on the contact layer 1002, whereas a p-type electrode 1008 is formed on the contact layer 1006.
Part of light emitted by the active layer 1004 that travels toward the n-type cladding layer 1005 is extracted outside via the cladding layer 1005.
Light that travels toward the p-type cladding layer 1003 is reflected by the multilayered reflective film 1001, and extracted outside via the n-type cladding layer 1005.
In this structure, light emitted toward the substrate 1000 can be reflected by the reflective film 1001, and extracted outside.
However, the reflectivity of light which is not vertically incident on the reflective film 1001 is low, the electrodes 1007 and 1008 which shield light exist on the light extraction surface, and the active layer 1004 is formed on the reflective film 1001. This results in low crystallinity and short service life.
FIG. 27 shows still another semiconductor light-emitting element relating to the present invention. An n-type InGaP buffer layer 1102, n-type InAlP cladding layer 1103, InGaAlP active layer 1104 functioning as a light-emitting layer, p-type InAlP cladding layer 1105, and p-type GaAs contact layer 1106 are formed on the upper surface of an n-type GaP substrate 1101. A p-type electrode 1107 is formed on the p-type GaAs contact layer 1106, while an n-type electrode 1100 is formed on the lower surface of the substrate 1101.
Light emitted by the InGaAlP active layer 1104 is reflected by the n- and p-type electrodes 1100 and 1107, and extracted outside from a region of the contact layer 1106 which is not shielded by the p-type electrode 1107.
In this structure, however, light concentrated immediately below the electrode 1107 is shielded by the electrode 1107, and cannot be extracted outside.
In the element shown in FIG. 27, only several % of light emitted by the active layer 1104 can be extracted outside owing to the difference in refractive index between the crystal and the air.
As the semiconductor light-emitting element, a compound semiconductor light-emitting element using a GaAs-based semiconductor material is adopted to emit light ranging from red to green, and a gallium nitride-based compound semiconductor light-emitting element using Al(x)Ga(y)In(1−x−y)N (0≦x, y≦1, x+y≦1) is adopted to emit light from the ultraviolet range to the blue/green range.
However, the refractive indices of these light-emitting elements are high (GaN=2.67, GaAs=3.62), their critical angles are small (GaN=21.9°, GaAs=16.0°), and thus their light extraction efficiencies are low.
The GaAs system exhibits large light absorption on the substrate. Emitted light is absorbed by the substrate to decrease the light extraction efficiency.
FIG. 29 shows still another semiconductor light-emitting element relating to the present invention.
An n-type GaAs buffer layer 1301, n-type InGaAlP cladding layer 1302, InGaAlP active layer 1303, p-type InGaAlP cladding layer 1304, and p-type AlGaAs current diffusion layer 1305 are sequentially grown on the upper surface of an n-type GaAs substrate 1300. A p-side electrode pad 1307 is formed on the p-type AlGaAs current diffusion layer 1305, whereas an n-side electrode 1306 is formed on the lower surface of the n-type GaAs substrate 1300.
In this structure, a current flowing from the p-side electrode 1307 is widened by the p-type AlGaAs current diffusion layer 1305, and injected from the p-type InGaAlP cladding layer 1304 to the InGaAlP active layer 1303. The light is extracted outside the element via the p-type AlGaAs current diffusion layer 1305.
In the GaAs-based compound semiconductor light-emitting element having this structure, part of light emitted by the active layer 1303 that travels toward the substrate 1300 is absorbed by the substrate 1300, and cannot be extracted outside the element. More specifically, 50% of the emitted light cannot be extracted, which is fatal to high luminance.
As described above, the elements relating to the present invention suffer low light extraction efficiency.